The Quandary of BOM's

July 2021
THE
QUANDARY OF
BOMS

2
Capgemini Invent 2020. All rights reserved |
The Quandary of BOM’s| Joe Herron & John Lopez | July 2021
One of the things we often run into in engineering and
manufacturing is the need for a descriptive configuration for
manufacturing and engineering illustrates “parts of a
whole” in two high level ways, structure, and
interaction. The classic bicycle example can be decomposed
into major sub-systems like frame, drive train, front-wheel
assembly and back-wheel assembly. Each sub-system can be
further broken down into individual piece parts
One important concept is the lowest level of configurable parts
in any assembly is the physical components, and the
assembly itself is a logical configuration of those
components. You could take the same parts and make
something different from the original specification!
So, let’s delve into the idea of an assembly and how a BOM (bill
of material) relates.
P A R T S O F A W H O L E

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BOM Stands for ‘Bill of Materials’
 “A bill of materials or product structure (sometimes bill of material, BOM or
associated list) is a list of the raw materials, sub-assemblies, intermediate
assemblies, sub-components, parts, and the quantities of each needed to
manufacture an end product.”
 https://en.wikipedia.org/wiki/Bill_of_materials
More Simply Put….
 a specification of all components and their hierarchical interrelation to
each other to create the functional assembly. It is a list of parts and how
they are structured together
, and in what order
, to build the desired
product. Remember Christmas time, those dreaded words: “some
assembly required”. Well, you’d never get there without detailed parts-
lists and assembly instructions!
Therefore, a BOM is like a report of the part structure(s) to
create the assemblies
 At the end of the day there can be many kinds of BOMs that are purpose-driven,
defining what is (and is not) included in the information, how many layers of
assembly are exposed, if it is simply to validate you have all the parts, or
specification of how to build a complex assembly using primary and alternate
parts or based on consumer facing configured product options.
W H A T I S
A B O M ?

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The Quandary of BOM’s| Joe Herron & John Lopez | July 2021 4
W H A T I S A B O M ?
Therefore, a BOM is like a report of the part
structure(s) to create the assemblies
 At the end of the day there can be many kinds of BOMs
that are purpose-driven, defining what is (and is not)
included in the information, how many layers of assembly
are exposed, if it is simply to validate you have all the
parts, or specification of how to build a complex assembly
using primary and alternate parts or based on consumer
facing configured product options.
 As a sidenote, since there are many kinds of BOMs as
defined by their purpose, usually a letter preceding the
acronym BOM is used to identify the intent of the BOM:
EBOM (engineering BOM), MBOM (manufacturing BOM),
etc. Generically when speaking of all-purpose driven BOM
usages, the term XBOM is used, where X defines a place
holder for the purpose driven BOM designation.
EXAMPLE: BICYCLE MODEL XYZ
• Frame
• Frame
• Forks
• Stems
• Head parts
• Braking Assembly
• Brake lever
• Front Brake
• Rear Brake
• Drive Train/Transmission
• Cranks
• Chain Wheel
• Pedal
• Chain
• Freewheel
• Control
• Handlebar
• Break

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W H Y U S E A B O M R A T H E R T H A N
A P A R T S - L I S T ?
In a way, a BOM is a parts-list. When all the parts, no matter how they are
structured are listed in a report, this is often called a “flat BOM”. The purpose of
a flat-BOM is simply to validate that when you get an assembly or a kit, that all the
required parts are accounted for.
However, the idea of a BOM (vs. a simple parts list) is that it provides context and
structure for the parts represented in the list. A flat-BOM assures that I have all the
parts I need, but it tells me nothing of how they function together (EBOM) or how to
perform the assembly operations (MBOM/ABOM). Back to the bicycle example: you
cannot assembly the front wheel braking parts until the wheel has all the spokes, hub,
and rim assembled, and is mounted on the steering assembly. Order and structure
matters.

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W H A T K I N D
O F B O M ’ S
A R E T H E R E ?
Since a BOM is a parts list, showing the
parts structured in a logical fashion to
create a product, there can be several
needs defined within a BOM. These are
purpose driven BOMs can be single
layered (just showing the parts or sub-
assemblies at the next level down) or
multi-layered (showing a hierarchy of
parts and sub-assemblies in a single
BOM report).

T Y P E S O F B O M S
Design Bill of material, specifies CAD Specific parts
required for modeling purposes such as sketch lines,
profiles, reference geometry, etc. BTO (Build to Order),
MTO (Make to Order), MTS (Make to Stock), and
CTO (Configure to Order) product definition is done
around part items and the CAD data is there to
supplement the product definition and aid in
manufacturing. Companies that plan to move to Build to
Order or Configure to Order should not invest too much
in CAD Data Configuration as it creates roadblocks for
the future.
DBOM/CAD BOM EBOM/MBOM
The EBOM is an engineering specification of all
components and their hierarchical interrelation to
each other to create the functional assembly
The MBOM is a Manufacturing derivative of the
EBOM, but whose purpose it is to specify how the
engineering specification is to be built. Production
Engineering (manufacturing engineering) will tear
the engineering assembly apart, possibly into
multiple MBOMs so that they can plan the build,
likewise for multi-operation build. Additionally, all
the required manufacturing and/or assembly
related items are listed. This would also include
items not listed in EBOM; adhesives, lubricants,
software, additional MBOM Structures if needed.
Multiple states of the pre-finished, intermediate
stages and finished states of the parts are
represented in the structure (rough, semi-finished,
and finished).

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ABOM – Assembly BOM an assembly
specific BOM for use on the assembly
floor
125 % BOM – Bill of material that has
substitutes or alternates for part
component or assembly levels
Neutral BOM – Bill of Material used as
a baseline configuration of a given
product with all available options, used
to generate/create Serialized As-
Maintained BOM at rev “00”

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“Overloaded BOM” (EBOM or MBOM) has all the main
parts, and alternates, or possible alternate modules to be
used in building out the assembly. A trivial example is
having an option of using a grade 5 or grade 8 bolt, both
having unique part numbers. More interestingly is the
DCAC structures that specify a “SCML” (supplier custom
module list) calling out the approved SMPLs (supplier
module parts lists) on a per Tail Number basis for
manufacturing delivery of suppliers. The overall airframe,
landing gear, etc. EBOM will have all options, but in
response to the SCML, only specific options are used.
Hence a 150% EBOM & MBOM will result in a 100% SCML
BOM As-Delivered.
This level of BOM would support business use cases
around Options and Variants i.e. the ability to “Configure
the end product” based on rule selection based on product
offerings.
150% BOM Unitized / Serialized As-
Maintained BOM
This BOM starts with a serialized version of the As-
Built/As-Shipped BOM that is specific to a given
serialized assembly. However, not only are the
manufacturing part numbers maintained in this
BOM, but so are the actual serial number parts that
physically exist in the build. Hence when an MRO
(maintenance, repair, and overhaul) activity occurs
which results in the replacement of parts – with
potential use of alternate part numbers – and new
serialized parts – then the UBOM is updated to
reflect the currently maintained configuration on a
serialized aircraft (tail number). Hence one can
query the UBOM at any time and know exactly what
part numbers and serial numbers of those parts are
in-service. This is a fleet maintenance BOM.

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As-Built BOM – This BOM is a derivative of the
MBOM where there are options/alternates that may
be chosen from the overloaded configuration and is
built with a given 100% configuration. This may be
a single assembly or a lot (500 wheel & brake
assemblies sold to a OEM all with the same
alternate configuration), this includes any deviations
that occurred to the product during
manufacturing/assembly processes from within MES
As-Shipped BOM – this is the As-Built, with
potential last-minute alterations, still within
specification for the customer

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Anyone responsible for validating a
design of a product, designing the
processes to create or buy parts that
go into the product, or maintaining the
product to the end-of-life. Everyone for
the most part needs some kind of
xBOM structure to understand, build,
and maintain the product.
W H O U S E S
A B O M ?

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W H E N I S A B O M
N E E D E D ?
Various XBOMs are used throughout the design, requirements
& specifications, manufacturing, and customer consumption
phases of a project. The XBOMs are authored/consumed by
persons in engineering, sales, manufacturing engineering, assembly,
and MRO (maintenance, repair, and overhaul) of the product(s)
created. The engineering EBOM may be the product of a CAD
(computer aided design) systems assembly structure or CAD-
BOM/DBOM.
The EBOM may be then consumed by manufacturing
engineering to design the processes required to make/buy the
product parts, required tooling, and fixturing to machine
components in-house or via suppliers, etc. This use of the MBOM
provides the folks who make the products the specifications
they need to build the products. The As-Built BOM may be a
customer specific BOM as they build the assembly in the
shop, based on use of alternate parts and possible options added-to
or taken-away-from the original EBOM.

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B O M A S A M O D E L O F T H E P R O D U C T
In a way, no matter which BOM(s) you must interact with, it is a logical model of the
product. That is a critical understanding in that many organizations are moving to
Model-Based Enterprise (MBE) operations architectures that will allow them to better
utilized the products of engineering and manufacturing to create must more efficient and
connected methodologies to service the product from inception, through production, into
in-flight operational feedback for predictive analysis of the performance of the product
and maintenance, through the end of the product lifecycle. It is the evolution of
engineering for and in manufacturing that is driving this. The old paradigm of
“over the wall” with the products of engineering is the nemesis of MBE!

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U N I T E D ,
W E S T R U C T U R E
Within Discrete Manufacturing when there is a need to Design,
Engineer and Manufacture a Product, why not take a new
approach that limits total number of structures and unite into
one Combined Bill of Material Structure? This approach eliminates
the need to have multiple structures changing out of synch of one
another. Collaboration takes place within one place and users can view
the Combined BOM Structure in different ways depending on their role
and needs. For an example, if the Design Engineer is requiring a CAD
Modeling perspective, they would like to view the structure without any
of the Manufacturing Stage Models displayed.
What about Manufacturing perspectives? A user that is responsible for
the Manufacturing Stage Models might need to hide all of the CAD only
related structure in order to focus on creating each one of the model
representations required in order to manufacturing the final piece-part.
Engineering may only need to view the structure with no Manufacturing
information to gauge what the BOM Structure would look like as it
relates to a parts list that defines all the Make vs. Buy levels. This new
approach seems arbitrary, however, when people can work
collectively within one Bill of Material, all pieces seem to come
together.

The Quandary of BOM's

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